The present invention relates to an objective lens for an optical pick-up of an optical disc drive for recording information to and/or reproducing information from an optical disc, such as a blu-ray disc, having a higher recording density than a DVD.
New-standard optical discs, such as a BD (Blu-ray Disc), having still higher recording density than DVD, are being brought into practical use in recent years. In general, such an objective lens is configured to converge a laser beam having a wavelength of approximately 400 nm. Objective lenses for a blu-ray disc are disclosed, for example, in Japanese Patent Provisional Publications Nos. 2003-114383A (hereafter, referred to as document #1) and 2005-156719A (hereafter, referred to as document #2). Each of the objective lenses disclosed in these publications is a single-element lens made of glass.
In order to configure an objective lens to form a more suitable beam spot for information recording/reproducing of an optical disc, it is required to increase a rim intensity (i.e., a ratio of an intensity of light passed through a perimeter of the objective lens to an intensity of light passed through a part in the vicinity of an optical axis of the objective lens). Each of Japanese Patent Provisional Publications Nos. 2004-39161A (hereafter, referred to as document #3) and 2005-11494A (hereafter, referred to as document #4) describes that a rim intensity can be increased by forming an anti-reflection coating on a lens surface of the objective lens.
The objective lenses disclosed in the documents #1 to #3 are glass lenses. A glass lens has a drawback that it is heavy in weight and expensive. By contrast, a plastic lens does not have such a drawback. However, in order to give the same refractive power as that of a glass lens to a plastic lens, a radius of curvature of each surface of the plastic lens inevitably decrease particularly on a light source side. As a result, the gradient of a light source side surface of the plastic lens becomes large in the perimeter of the light source side surface, and thereby the manufacturability of the plastic lens deteriorates.
More specifically, a plastic lens is manufactured by injection molding through use of a mold, and the mold is made by cutting a metal piece with a bite while rotating the metal piece using a turning machine. If the metal piece has to be cut to have a steep surface, accurate cutting becomes difficult.
It becomes also difficult to measure accurately a condition of a surface of a manufactured plastic lens or a processed surface of a mold with a contact probe because a steep portion of the surface of the plastic lens or the mold has a large changing amount of sag with respect to a distance from a center (of the lens or the mold) and therefore the measured sag amount changes considerably even when a positional error of the probe in a radial direction is very small.
In addition, coating of such a lens surface having a steep portion is difficult. That is because if the gradient of a lens surface on which substance of vapor deposition is to be adhered is large, adhesion of the substance of vapor deposition to the lens surface deteriorates. In this case, the uniformity of the thickness of the coating may deteriorate, or the uniformity of the refractive power of the plastic lens may deteriorate. Furthermore, if the gradient of a lens surface is large, transmissivity of the lens may decrease and thereby a property (e.g., a jitter amount) of an optical disc drive having the lens may be badly affected.
The objective lens disclosed in document #3 is made of glass and is provided with a single layer coating. However, if the material of the objective lens is plastic, it becomes impossible to secure a relatively high rim intensity because in this case a difference in refractive index between the material of the objective lens and the coating is small and therefore adequate performance of anti-reflection can not be achieved.
The coating disclosed in document #4 is configured such that an maximum wavelength of a light beam incident on the objective lens at an incident angle of 0 is set at 680 nm. However, in this case, the thickness of the coating is inadequate at the perimeter of the objective lens and it becomes impossible to increase transmissivity. Consequently, it becomes impossible to increase the rim intensity.